1. Field of the Invention
This invention relates to growth of epitaxial layers and, more particularly, to the growth of garnet epitaxial layers.
2. Art Background
Magnetic bubble wafers are presently being used in memory devices for various commercial applications. Of the numerous steps required to produce a finished device, one of the most significant in determining device properties is the growth of an epilayer of magnetic garnet material on an appropriate substrate. Since commercial use of magnetic bubble devices appears to be increasing, efficient epitaxial growth methods are essential. Unfortunately, difficulties are encountered in the growth of epilayers even when processing a single substrate. These difficulties remain and indeed are considerably magnified by an attempt simultaneously to coat a plurality of substrates.
The predominate difficulties associated with the growth of an epilayer on a single substrate are mesa and microcrystalline formation. The formation of these defects substantially degrade the quality of the ultimate magnetic bubble device. Mesa defects are irregularities in the epilayer which typically project between 0.2 .mu.m and 20 .mu.m above the epilayer surface. Microcrystals are crystalline phase discontinuities on the surface or in the body of the epilayer having essentially the same composition as the epilayer and usually having dimensions of 0.5 .mu.m to 50 .mu.m.
Various processes are utilized to avoid formation of these defects. The most common method of limiting mesa formation is by spinning the substrates shortly after they emerge from the melt. Although spinning has had moderate success, mesa defects located near the center of the substrate are not substantially affected. Attempts have been made to reduce mesa formation further by growing the epilayers with the substrates at an angle to the surface of the melt. It was hypothesized that when the epitaxial layers were grown at a slight tilt (5 degrees) and the substrates emerge from the melt at this same tilt subsequent spinning would be more effective in reducing mesa formation, Gies et al, IBM Technical Disclosure Bulletin, 16 (9), 3049 (1974). This approach, however, causes serious non-uniformity in the epilayer thickness on each substrate which is far more disadvantageous than mesa formation. Irrespective of the cause of mesa formation they produce a lowered yield of acceptable devices.
The second possible serious defect, microcrystalline formation, depends on the processing condition, such as the degree of supercooling of the melt. Attempts to reduce microcrystalline formation usually involve adjusting the process parameters. For example, the substrates are preheated before immersion in the growth melt to prevent formation of crystals in the melt which ultimately are incorporated in the epilayer. Additionally the melt is carefully thermally re-equilibrated between growth runs to avoid microcrystals. When one substrate or only a few substrates are processed in a single run it is impractical, if a satisfactory production rate is to be achieved, to take these time consuming precautions (such as re-equilibration between runs). Therefore microcrystalline defects usually persist in one substrate processing and result in a lowering of the yield of acceptable devices.
In an attempt to increase efficiency and to make feasible the time consuming precautions against microcrystalline formation which are unacceptable during single substrate deposition, multiple substrate processing has been investigated. The simultaneous processing of a number of substrates, however, magnifies the problems encountered with single substrate processing and additionally introduces new difficulties. When a multiplicity of substrates are processed, they are stacked horizontally in a column with an average spacing between growth surfaces of about 1 cm. When the column is inserted in the melt, these spaces confine the flow of the growth melt around the substrate and in turn increase the amount of melt remaining on the eiplayer when it is extracted from the growth media. After extraction the additional melt continues localized growth of the epilayer and produces mesas. The spinning method is also employed in multiple substrate processes to eliminate the extra melt and the resultant mesas. Nevertheless, the additional melt left upon extraction from the melt body naturally leads to an increase in mesas remaining after spinning. Microcrystalline defects also appear to be more numerous when a plurality of substrates are being processed if process parameters are not closely controlled.
The increase of defects such as mesa and microcrystalline formation in multisubstrate processing is accompanied by other equally disadvantageous phenomenon. For example, an inordinate frequency of substrate breakage particularly for substrates in the lower section of the substrate column has been observed. This breakage problem causes the affected substrates to drop into the growth melt. The dissolution of the substrate in the melt changes the melt composition and precludes the further use of the melt for epitaxial layer growth.
The epilayer uniformity is also much more difficult to control in multiple substrate processing. Variations greater than 9% in thickness and 13% in magnetic properties between the top and bottom substrate of only an eight substrate column are often observed. (See Warren et al, American Institute of Physics, 19th Conference on Magnetism and Magnetic Materials, Boston, November 1973.)
The aggregation of the problems introduced by multiple substrate processing together with the magnification of problems normally associated with the growth of a single epitaxial layer substantially decreases the yield of useful epitaxial layers for device fabrication. This decreased yield in view of the substantial cost of the substrates is an obstacle to efficient, economic magnetic bubble device fabrication.